1. Field of the Invention
Embodiments of the invention relate to the field of device fabrication. More particularly, the present invention relates to a wafer handling end effector apparatus used to transport wafer materials among devices.
2. Discussion of Related Art
Silicon wafers are used in semiconductor and/or solar cell fabrication. The wafers are subjected to a multi-step manufacturing process that may involve a plurality of machines and a plurality of stations. Thus, the wafers need to be transported from one machine/station to another machine/station one or more times.
When transporting wafers from station to station or machine to machine in a manufacturing process, apparatuses called end effectors are typically employed. A typical end effector apparatus may be hand-like in appearance. A base unit may attach to a plurality of finger-like extensions. On each of the finger-like extensions, a plurality of wafers may be seated atop wafer pads at spaced apart intervals. The end result may be a matrix of wafers supported by the plurality of end effector fingers. The end effector may typically be moved linearly (e.g., forward and backward) as well as rotationally all in the same plane (e.g., x-y axis). The end effector may also be moved in a third direction along a z-axis to provide a full range of motion.
There are several styles of wafer interface features used in semiconductor wafer handling equipment. Sometimes silicone pads are used to isolate the silicon wafer from the metallic robot end effector fingers to prevent contamination of the wafer by the metallic arm. Sometimes wafers sit on hard slippery pads affixed at periodic intervals along the end effector fingers. The hard slippery pads are typically made of a polyetheretherketone (PEEK) plastic. PEEK is a high performance thermoplastic material comprised of a polymer that is semi-crystalline. PEEK polymer is advantageous in that retains its mechanical properties at extremely high temperatures, is weldable, machinable, and can be bonded with epoxies cyanoacrylates, polyurethanes, or silicones.
One of the characteristics of an end effector apparatus is its rigidity. An end effector apparatus should be rigid enough to withstand the forces that are applied to it during normal operation. An end effector apparatus should be rigid enough to operate at higher speeds. Higher speeds translate to greater product workflow as measured by the number of wafers that can be handled per hour (WPH) by a wafer handling robot. Increasing the WPH allows the wafer handling robot to process more wafers leading to a more productive and efficient process.
End effector apparatus stiffness or rigidity can impact the speed at which the wafer handling robot can operate and consequently, impact the number of wafers that can be processed (e.g., WPH). One measurement or characteristic of an end effector apparatus is its natural frequency. Natural frequency is the frequency at which a system naturally vibrates once it has been set into motion. In other words, natural frequency is the number of times a system will oscillate (move back and forth) between its original position and its displaced position, if there is no outside interference. Resonance is the buildup of large vibration amplitude that occurs when an object is excited at its natural frequency. Undesirable mechanical resonance can cause bridges to collapse, aircraft wings to break, and machinery to break or malfunction.
The natural frequency of the end effector apparatus is one factor that contributes to the overall stiffness and system stability. The higher the natural frequency, the stiffer the end effector apparatus, thus the more stable the wafer handling system. For example, typical end effector apparatuses exhibit a natural frequency of approximately 20 to 27 Hz and systems using such an end effector apparatus can process approximately 1700 wafers per hour (WPH). Another factor affecting WPH may be the mass of an end effector apparatus. Minimizing mass while maximizing the natural frequency leads to an optimized end effector apparatus for throughput and stability purposes as measured by WPH.